Sickle cell disease (SCD) is associated with severe pain, which remains a major challenge to treat. Opioids are the current standard of care, but due to side effects and development of tolerance and dependence, remain a sub-optimal approach to treat SCD pain, particularly when used on a continued basis. Sickle patients therefore live with recurrent difficult-to-treat pain, resulting in frequent hospitalization and loss of productivty. Thus there is a critical need to develop effective therapies devoid of addiction and tolerance to treat severe pain in SCD. We have recently demonstrated that nociceptin receptor (NOP) agonists have significant anti-nociceptive and anti-allodynic effects in an animal model of chronic pain, with efficacies comparable to that of the opioid morphine, in the same model. We further demonstrated that bifunctional NOP/opioid agonists also possess anti-allodynic activity, and do not develop tolerance and have no rewarding effects on their own. The nociceptin receptor NOP and its endogenous ligand nociceptin/orphanin FQ (N/OFQ) are widely distributed in the brain and spinal cord in regions involved in nociceptive responses and are a target for pain therapeutics. The NOP system also modulates opioid effects such as anti-nociception, opioid-induced tolerance and reward. Therefore, from a therapeutic standpoint, NOP receptor agonism appears to be a broad, promising pharmacological strategy to provide pain relief in sickle cell pain, without the liabilities associated with opioid- based therapies, such as constipation, tolerance and dependence. This application therefore proposes proof- of-concept studies to investigate the efficacy of NOP agonists in the treatment of chronic pain associated with sickle cell disease, with the following Specific Aims: Aim 1: To identify one NOP agonist and one bifunctional NOP agonist/mu partial agonist, which have suitable drug-like characteristics and brain penetration, for evaluation in efficacy studies. Aim 2: To determine the analgesic ability of NOP agonist and NOP agonist/mu partial agonist on pain in sickle mice. We will examine the effect on, (a) tonic hyperalgesia representing chronic pain, (b) acute pain due to vaso-occlusive crises incited by hypoxia/reoxygenation, and (c) inflammation and organ pathology to rule out the adverse effects of NOP-based analgesics. We will use relevant mouse models of sickle cell disease recently developed in our laboratory and employ several different measures of pain including testing for cutaneous and deep tissue hyperalgesia and sensitivity to heat and cold, which will provide comprehensive investigation on different pain characteristics observed clinically in patients with SCD. These studies will facilitate the translation of the promising efficacy of NOP receptor agonists in chronic pain into therapeutic use to expand the options for pain relief in SCD, without inadvertent opioid-related side effects. Promising, efficacious NOP agonists identified from this project can be further developed as potential pharmacotherapy for the treatment of pain in SCD. PUBLIC HEALTH RELEVANCE: Patients with sickle cell disease (SCD) suffer with severe pain throughout life, which is challenging to treat. Opioids are the only therapy widely used but unfortunately are associated with side effects (constipation) and development of tolerance and addiction. Nociceptin receptor (NOP) agonists are able to produce opioid-level analgesia without the opioid-related side effects, such as tolerance and dependence, in models of pain. If they demonstrate proof-of-efficacy in the animal models of SCD, as proposed in this project, they would offer a tremendous therapeutic advantage to treat pain in SCD without opioid-related side effects.